When the brakes are applied on a vehicle traveling at a given velocity, braking torques are generated at each of the brake wheels. The braking torque causes a retarding or braking force to be generated at the interface between the tire and the surface. The braking forces generated at the wheels then cause a decrease in the vehicle velocity.
Ideally, the braking forces at the wheels increase proportionately as the driver increases the force on the brake peddle. Unfortunately, this is not always the case in braking procedures. As the braking torque, and hence the braking force at the wheel, is increased, the rotational speed of the braked wheels becomes less than the speed of the vehicle. When the rotational speed of the wheel is less than the vehicle speed, "slippage" is said to occur between the tire and the surface. With further increase in brake pressure, the slippage between the tire and the surface increases until lockup and skidding of the wheel occurs. In most cases, lockup causes a reduction in braking force and increase in stopping distance. Lockup also causes a degradation in directional control due to a reduction in the lateral forces at the wheels.
Both of these problems associated with lockup were addressed with the advent of anti-lock brake systems (ABS). The basic anti-lock brake system monitors the speed at each of the wheels, decides whether the wheel is excessively slipping based on these speed measurements, and modulates the braking pressure accordingly to avoid lockup. The ABS aids in retaining vehicle stability and steerability while providing shorter stopping distances.
One method by which a state of excessive slippage is identified in the ABS includes comparing the speed of each wheel to a reference speed. The reference speed is an estimate of the true vehicle speed based on current and previous values of the individual wheel speed. If the speed of a wheel is significantly less than the reference speed, then the wheel is deemed by the ABS to be excessively slipping. The ABS then reduces the pressure actuating the brake in order to reduce brake torque. The reduction of brake torque allows the friction force at the surface to accelerate the wheels, thereby causing a reduction of the slip in the wheels.
After a period of constant braking pressure following the pressure reduction, the pressure actuating the brake is increased until excessive wheel slip occurs again. The cycle of decreasing the pressure, maintaining constant brake pressure, then increasing brake pressure is repeated until the anti-lock event ends. The parameters which define the specifics of this cycle depend on both the vehicle and the surface conditions.
For the present invention, the braking of a vehicle on a surface with varying coefficient of friction is considered. The coefficient of friction, mu, of a surface is defined as the ratio of the braking force generated at the interface between the tire and the surface to the normal force between the tire and the surface.
Three classes of surfaces can be defined qualitatively in terms of mu: high mu, low mu, and split mu. A high mu surface is one which produces relatively good braking ability. Dry asphalt is an example of a high mu surface. A low mu surface is characterized by its resulting in poor braking ability. An example of a low mu surface is a road covered with snow or ice. A split mu surface is encountered when a vehicle has some of its tires on a low mu surface and the other tires on a high mu surface. An example of a split mu surface is a road with snow or ice on one side of the vehicle and dry asphalt on the other side of the vehicle.
An example of a vehicle braking on a split mu surface is one in which one side of the vehicle is braking on an asphalt (high mu) surface, while the other side of the vehicle is braking on ice (low mu). The difference in braking torque from side to side induces a yaw moment which may be sufficient to cause vehicle instability and loss of control. The driver is then required to provide corrective steering measures to maintain the intended direction of the vehicle.
Known prior art systems typically estimate the mu of a surface based on the vehicle deceleration. Therefore, on a split mu surface, the estimated mu is an average of a high mu surface and a low mu surface. As a result, the mu is underestimated for the wheel braking on the high mu surface, while it is overestimated for the wheel braking on the low mu surface. It is common in the art to use the estimate of the surface mu to influence the pressure apply control to the brakes of the wheels. By underestimating the mu for the wheel braking on the high mu surface, insufficient brake pressure is initially applied to the wheel resulting in under-utilization of the surface adhesion. The overestimation of the mu for the wheel braking on the low mu surface results in too much pressure being applied to the low mu wheel; thereby forcing instability of the wheel.
The need exists within an anti-lock brake system for reducing the instability of a vehicle braking on a split mu surface due to an incorrect estimate of mu.